Should All Power Cables be Alike?

It has entered my limited consciousness that there lies the need to change ALL power cords if you change one; but I don't know the answer. For example, would upgrading only the AC power cord to a power conditioner or generator be limited improvement if the components feeding into the power unit weren't also upgraded? Another words, if changing only the primary AC cable (i.e., the one from the AC wall plug) is one sub-optimizing improvement without also changing the AC cord from all the components in the system?